Cadmium tolerance and accumulation characteristics of mature flax, cv. Hermes: contribution of the basal stem compared to the root.

The potential of mature flax plants (cv. Hermes) to tolerate and accumulate cadmium (Cd) was studied to determine which part of the plant would be the key organ for phytoremediation purposes. After 4 month-growth on sand substrate containing 0.1mM Cd in a greenhouse, the roots and stems were separated and the stems were divided into three parts. The effects of Cd were studied on growth parameters, histology and mineral nutrition. No visible toxic symptoms were observed. Tolerance-index values calculated from growth parameters and nutrients remained relatively high, allowing the development of the plant until maturity and formation of seeds. The roots and bottom stem accumulated the highest quantity of Cd (750 and 360 mg/kg dry matter), values which largely exceeded the threshold defined for hyperaccumulators. On the other hand, basal stem had a high bioconcentration factor (BCF=32) and translocation factor TF' (2.5) but a low TF (0.5), indicating that this basal part would play a major role in phytoremediation (phytostabilization rather than phytorextraction). Therefore, the high tolerance to Cd and accumulation capacity make possible to grow Hermes flax on Cd-polluted soils.

Impact of cadmium on early stages of flax fibre differentiation: ultrastructural aspects and pectic features of cell walls.

The effect of 0.5mM cadmium (Cd) was studied on the ultrastructural aspects and pectin features of the walls of flax cellulosic fibres when the thickening of secondary wall had just started in the hypocotyl of 10-day old seedlings. As seen by PATAg staining in controls, cell-wall formation displayed two distinct steps, secretion and remodelling, which did not occur simultaneously for all the neighbouring fibres. The inner part of the secondary wall, where the cellulose molecules had just been synthesized, appeared very reactive to PATAg. The outer part, where the cellulose fibrils associated in larger microfibril complexes, became non-reactive to PATAg. Under Cd treatment, we noticed some acceleration of fibre differentiation in terms of fibre number, wall thickness and yield. As revealed by PATAg staining, treated fibres exhibited a disturbed cell-wall texture, indicating a modified adhesion between the matrix polysaccharides and the cellulose microfibrils. The Cd impact on the distribution of highly methylesterified homogalacturonans (recognized by JIM7 antibody) was moderate in the cell junctions and low in the primary wall and outer part of secondary wall. The data meant that no early deesterification occurred in these domains, a behaviour related to the specificity of the CW-II metabolism. No large redistribution of low esterified homogalacturonans (recognized by JIM5 antibody) happened either. In parallel, the amount of uronic acid significantly increased in the so-called H(2)SO(4) cell-wall extract, indicating a Cd impact on pectin structure not detected by JIM5 or JIM7 antibodies.

Characteristics of cadmium tolerance in 'Hermes' flax seedlings: contribution of cell walls.

Most flax (Linum usitatissimum) varieties are described as tolerant to high concentrations of Cd. The aim of the present paper was to better characterize this tolerance, by studying the responses of flax plantlets, cv Hermes, to 18d growth on 0.5mM Cd. In Cd-treated seedlings, the majority of Cd was compartmentalized in the roots. Analysis of other elements showed that only Fe concentration was reduced, while Mn increased. Growth parameters of Cd treated flax were only moderately altered, with similar mass tolerance-indices for roots and shoots. Tissue anatomy was unaffected by treatment. The effect on lipid peroxidation, protein carbonylation and antioxidative activities appeared low but slightly higher in roots. The most important impacts of Cd were, in all organs, cell expansion, cell-wall thickening, pectin cross-linking and increase of cell-wall enzymatic activities (pectin methylesterase and peroxidase). Thus, the role of the cell wall in Cd tolerance might be important at two levels: (i) in the reinforcement of the tissue cohesion and (ii) in the sequestration of Cd.

Spatial regulation of cell-wall structure in response to heavy metal stress: cadmium-induced alteration of the methyl-esterification pattern of homogalacturonans.

BACKGROUND AND AIMS: In flax hypocotyls, cadmium-induced reorientation of growth coincides with marked changes in homogalacturonan (HGA) epitopes that were recognized by JIM7 and JIM5 antibodies in the external tangential wall of the epidermis. In the present study, LM7 and 2F4 monoclonal antibodies were used, in addition to JIM5 and JIM7, to extend the investigation on the methyl-esterification pattern of HGA within various domains of the cortical tissues, including the cortical parenchyma where cell cohesion is crucial. METHODS: The PATAg (periodic acid thiocarbohydrazide-silver proteinate) test was applied to ultrathin sections so that the polysaccharides could be visualized and the ultrastructure studied. The monoclonal LM7, JIM5 and JIM7 antibodies that recognize differently methyl-esterified HGA were used. The monoclonal 2F4 antibody that is specific to a particular polygalacturonic acid conformation induced by a given calcium to sodium ratio was also applied. After immunogold labelling, the grids were stained with uranyl-acetate, the samples were observed using a transmission electron microscope and the gold particles were counted. KEY RESULTS: In the presence of cadmium, the increase of LM7 labelling in external tangential wall of the epidermis, together with a decrease of JIM7 labelling, suggested a specific role for randomly partially de-esterified HGA to counteract the radial swelling stress. Enhanced JIM5 and 2F4 labelling in the junctions of the inner tissues indicated that the presence of blockwise de-esterified HGA might oppose cell separation. CONCLUSIONS: The response of the hypocotyl to cadmium stress was to adapt the structure of the wall of cortical tissues by differently modulating the methyl-esterification pattern of HGA in various domains.

Temporal regulation of cell-wall pectin methylesterase and peroxidase isoforms in cadmium-treated flax hypocotyl.

BACKGROUND AND AIMS: In hypocotyls of flax (Linum usitatissimum) cadmium-induced reorientation of growth (i.e. an increase in expansion and a decrease in elongation) coincides with marked changes in the methylesterification and cross-linking of homogalacturonans within various cell-wall (CW) domains. The aim of the present study was to examine the involvement of pectin methylesterase (PME) and peroxidase (PER) in this cadmium-induced CW remodelling. METHODS: CW proteins were extracted from hypocotyls of 10- and 18-d-old flax that had been treated or not treated with 0.5 mm Cd(NO(3))(2). PME and PER expression within these extracts was detected by LC/MS, by isoelectric focusing and enzyme activity assays. Transcript expression by RT-PCR of known flax PME and PER genes was also measured in corresponding samples. KEY RESULTS: In cadmium-treated seedlings, PME activity increased as compared with controls, particularly at day 10. The increased activity of PME was accompanied by increased abundance of both a basic protein isoform (B2) and a particular transcript (Lupme5). In contrast, induction of PER activity by cadmium was highest at day 18. Among the four reported PER genes, Flxper1 and 3 increased in abundance in the presence of cadmium at day 18. CONCLUSIONS: The temporal regulation of Lupme and Flxper genes and of their respective enzyme activities fits the previously reported cadmium-induced structural changes of homogalacturonans within the CWs. After PME-catalysed de-esterification of homogalacturonans, their cross-linking would depend on the activity of PERs interacting with calcium-dimerized blocks and reinforce the cell cohesion during the cadmium-induced swelling.